All of them are excellent trainers. If Paul and Gary wasn't so self centered when they were his rivals and went on with him on Ash's journey, All of them would've learned alot more from training Pokemon and being a better person.

And as for AJ, he was an awesome Pokemon trainer who had a 100 to 0 win streak with only a Sandshrew. It's a shame they never brought him back after that one episode tho.